PIE's closest relatives

I have read (or re-read in one case) recently two books. J. P. Mallory's "In Search of the Indo-Europeans" judges that PIE was most closely related to Proto-Finno-Uralic. M. Ruhlen's "The Origin of Language" asserts that Proto-Semitic and Proto-Kartvelian are more closely related. As I approach the Indo-European question from primarily an archaeological and human perspective (only taken 1 course on linguistics so far and read a few books), I would be very interested to hear the educated opinions from this list on this subject.

 

A few digests ago the matter of a possible Semitic substrate across Europe came up. I guess the question may be, does this really exist, and is it a product of PIE contact with Semitic languages (early PIE - PSemitic relations) or over an overlay of Indo-European speakers over Semitic speakers (a true substrate). This seems like it would be an interesting debate and an important finding to determine.